A satellite based navigation device works by receiving and processing satellite signals from a global navigation satellite system such as the Global Positioning System (GPS), which consists of twenty-four satellites that orbit the earth and transmit signals that are received by the navigational device. A detailed description of GPS can be found in U.S. Government documents, Technical Characteristics of the Navstar GPS, June 1991, and GPS Interface Control Document, ICD-GPS-200, October, 1993.
Satellite based navigation devices have been used for many years to assist in navigation. Initially, GPS navigation devices were used for military applications. Today, in addition to military applications, satellite based navigation devices are used commercially and privately in automobiles, boats, and aircraft, for example. Unfortunately, satellite signals are susceptible to interference, both unintentional and intentional, due to the low power level of the satellite signals. Signals from cellular telephones and television transmitters, for example, may unintentionally interfere with satellite signals. However, intentional interference, i.e., jamming, may also interfere with the satellite navigation signals. When interference occurs, the performance of the navigation device may degrade.
To maintain or improve the performance of a satellite based navigation device in the presence of interference, a system for locating the source of the interference was developed by the Lockheed Martin Corporation and is described in U.S. Pat. No. 5,936,571 (“the '571 patent”), which is incorporated herein by reference. By knowing the location of the interference, the performance of the navigation device may be maintained or enhanced in the presence of the interference by, for example, avoiding the interference, i.e., planning a route that minimizes the impact of the interference, or eliminating the interference. The present invention utilizes anti-jam processing to enhance the performance of certain embodiments disclosed in the '571 patent.